New Axially Loaded Full Ring Test Method for Assessment of Susceptibility of Girth Welds and Parent Pipe to Sour Service Cracking Available to Purchase

ABSTRACT

Susceptibility to cracking in sour service is usually determined by testing in laboratory or simulated service environments, in compliance with NACE MR0175 / ISO 15156¹. Typically small scale specimens are extracted from sampled material to facilitate uniaxial tensile, C-ring or 4-point bend testing to determine the resistance to Sulfide Stress Cracking (SSC) or Stress Corrosion Cracking (SCC). Sampling relaxes residual stress and in many cases the specimen is not representative of the material surface condition and microstructure, and clearly may not represent material with inhomogeneous properties. This is particularly true for subsea lines installed by reeling as strain history varies around the entire circumference. A full ring ovalization test method was developed in the 1980s / 1990s, which retains the residual stresses, is better able to assess behavior with hoop stress and is still favored today (BS 8701²), but it does not load the entire specimen. This paper describes a new axially loaded full ring test method which was developed and demonstrated to combine the benefits of retaining a full as-welded pipe pup-piece, permitting single-sided exposure, with the advantage of tensile loading of the complete tubular specimen.

INTRODUCTION

Carbon steel girth welds for sour service applications are typically qualified using either ?small-scale? four point bent beam tests, to the requirement of NACE TM0177³ / EFC 16⁴ or ‘full-scale? full ring tests to OTI 95 635⁵ (BS 8701²). The four point bend test method can be un-representative of the material in service, since all faces of the specimens are exposed to the hydrogen-charging environment and the as-manufactured bore and weld root surface is often removed during the preparation operations. In addition the residual stresses from the pipe manufacture, welding and any subsequent processes are significantly reduced or removed. The ‘loss‘ of residual stresses is compensated for in NACE MR075 / ISO 15156¹ by applying stresses based on the actual yield strength (AYS) as opposed to the design stress, resulting in the requirement to stress specimens to 80% AYS for standard NACE TM0177- Solution A³ conditions or at 90% AYS for fitness-for-purpose conditions.